Linux embedded: misc improvements

- Allow static build
- Use .svg icons
- Linux Embedded lacks normal "open file dialog". So simply scan
  files in root folder of USB stick and return list.
- Change QProcess::execute(command) calls to
  QProcess::execute(command, args) calls to silence Qt 5.15.0
  depreciation warning.

imagewriter.h

@@ -75,8 +75,16 @@ public:
     /* Returns true if online */
     Q_INVOKABLE bool isOnline();
 
+    /* Returns true if run on embedded Linux platform */
     Q_INVOKABLE bool isEmbeddedMode();
 
+    /* Mount any USB sticks that can contain source images under /media
+       Returns true if at least one device was mounted */
+    Q_INVOKABLE bool mountUsbSourceMedia();
+
+    /* Returns a json formatted list of the OS images found on USB stick */
+    Q_INVOKABLE QByteArray getUsbSourceOSlist();
+
 signals:
     /* We are emiting signals with QVariant as parameters because QML likes it that way */
 
@@ -93,6 +101,7 @@ protected slots:
 
     void pollProgress();
     void pollNetwork();
+    void syncTime();
     void onSuccess();
     void onError(QString msg);
     void onFileSelected(QString filename);